This invention relates to a video duplication control system for playing back a video signal recorded on a recording medium, and limiting or preventing the reproduced video signal from being played back and recorded on another recording medium. It also relates to a video playback device, video recording device and video recording medium on which a video signal is recorded which use this video duplication control system.
VTR (Video Tape recording devices) are a commonplace feature of modern-day life, and many kinds of software exist which can be played back on a VTR. Digital VTR or DVD (Digital Video Disks) playback devices are now a reality, and provide videos and sound of exceptionally high quality.
There is, however, a problem in that this great abundance of software can be copied without restriction, and several methods have already been proposed to prohibit this.
For example, for a VTR which outputs an analog video signal, one method to prevent copying uses a difference in the AGC (Automatic Gain Control) system, or in the APC (Automatic Phase Control) system, of the VTR recording device and of a monitor receiver which displays the video.
When the VTR employs AGC using a pseudo sync signal inserted in the video signal, a monitor receiver employs AGC which does not use this pseudo sync signal. In this anti-duplication method using a difference in the type of AGC, a very high level pseudo sync signal is inserted and output in the video signal supplied from the playback VTR to the recording VTR as an AGC sync signal.
When the VTR employs APC using the phase of a color burst in the video signal, the monitor receiver uses a different type of APC. In an anti-duplication method using a difference in the type of APC, the phase of the color burst in the video signal supplied from the playback VTR to the recording VTR is partially reversed.
The monitor receiver which receives the analog video signal from the playback VTR plays back the video correctly without being affected by the high level pseudo sync signal in AGC or the partial phase reversal of the color burst signal in APC.
On the other hand, when a VTR records, on a recording medium, the analog video signal into which pseudo sync signals have been inserted or the analog video signal which has been subjected to color burst signal phase reversing control in the playback VTR as described hereabove, proper gain control or phase control based on the input signal cannot be performed, and so the video signal is not correctly recorded. Even if this signal is played back, therefore, normal picture and sound cannot be obtained.
In the case of a digitized video signal, e.g. in a digital VTR, an anti-duplication signal or an anti-duplication control signal comprising for example a duplication ranking control code, is added as digital data to the video signal and recorded on the recording medium, so as to prevent or control duplication of the video.
In this case, the playback digital VTR reads the video signal, audio signal and anti-duplication control signal, and supplies them as digital or analog data to a recording digital VTR.
In the digital VTR being used as a recording device, the anti-duplication control signal is extracted from the supplied playback signal, and recording of the playback signal is then controlled based on the anti-duplication control signal. For example, when the anti-duplication control signal comprises an anti-duplication signal, the recording VTR does not perform recording.
Alternatively, when the anti-duplication control signal comprises a copy ranking control code, recording is controlled by this ranking control code. For example, when the copy ranking code limits duplication to one copy, the digital VTR used for recording adds this anti-duplication code before recording the video signal and audio signal on the recording medium as digital data. It is thereafter impossible to duplicate the video signal from the copy.
Hence, in the case of a digital connection when the video signal, the audio signal, and the anti-duplication control signal used as digital signals are supplied to the digital VTR used as a recording device, anti-duplication control is performed on the recording side using the anti-duplication control signal by supplying this signal to the digital VTR as digital data.
However, in the case of an analog connection where the video signal and audio signal are supplied as analog signals, the anti-duplication control signal is lost when the signal supplied to the recording device is D/A converted. Hence, in the case of an analog connection, an anti-duplication control signal must be added to the D/A converted video or sound signal, and this causes deterioration of the video signal and audio signal.
It is, therefore, difficult to add an anti-duplication control signal and to extract it in the recorder for the purpose of anti-duplication control, without causing deterioration of the D/A converted video signal or audio signal.
Conventionally, therefore, in the case of an analog connection, duplication was prevented by an anti-duplication method using a difference in the AGC, or a difference in APC characteristics, between the VTR and the monitor receiver as described hereabove.
However, in some cases, when anti-duplication is prevented using the aforesaid difference in the AGC or a difference in APC characteristics between the VTR and the monitor receiver, depending on the type of AGC or APC characteristics on the recording side, the video signal may nevertheless be correctly recorded. In this case, it might happen that duplication cannot be prevented, or that the reproduced video on the monitor receiver is distorted. Further, it was troublesome to change over the anti-duplication method depending on whether there was an analog connection or a digital connection.
The inventors have already proposed an anti-duplication prevention method in which a spectrally spread anti-duplication control signal is superimposed on a video signal (U.S. patent application Ser. No. 08/175,510). This method may be used for both digital connections and analog connections, and it causes no deterioration of the video or sound which is played back.
According to this method, when the original recording medium is fabricated, a PN (Pseudorandom Noise) sequence code (referred to hereafter as PN code) used as a spread code is generated with a sufficiently fast period and spectrally spread by applying it to the anti-duplication control signal. In this way, a narrow bandwidth, high level anti-duplication control signal is converted to a wideband, low level signal which does not affect the video signal or sound signal. This spectrally spread anti-duplication control signal is then superimposed on the video signal supplied to the recording medium, and recorded.
On the recording side, a PN code is generated with the same timing and phase as the PN code used for spectral spread in the playback device relative to the video signal supplied by the playback device. The generated PN code is applied to the video signal on which the anti-duplication control signal is superimposed so as to extract the original anti-duplication control signal, i.e. so as to perform reverse spectral spread. Anti-duplication is then controlled based on the anti-duplication control signal extracted by reverse spectral spread.
In this way, the anti-duplication control signal is spectrally spread and superimposed on the video signal as a wideband, low level signal in the playback device. It is therefore difficult for a person wishing to illegally duplicate the video signal, to remove the anti-duplication control signal which is superimposed on it.
However, it is possible for a person aiming to prevent illegal duplication to detect the superimposed anti-duplication control signal by reverse spectral spread, and use it. This anti-duplication control signal is therefore supplied to the recording device together with the video signal. On the recording side, the anti-duplication control signal is detected, and duplication is precisely controlled according to the detected anti-duplication control signal.
According to this method, as described hereabove, the spectrally spread anti-duplication control signal is superimposed as a wideband, low level signal on the video signal, but it must be superimposed at a lower S/N ratio than that of the video signal in order for the video signal not to cause deterioration of the video signal.
To superimpose the spectrally spread anti-duplication control signal at a lower S/N ratio than that of the video signal, and to be able to detect the anti-duplication control signal superimposed on the video signal in the recording device, the number of the PN code (PN code length) required to spectrally spread a 1 bit anti-duplication control signal must be sufficiently large. The PN code length per bit of the anti-duplication control signal may also be expressed as a spread gain (spread factor) which is the ratio (T/TC) of a time width T per bit of the anti-duplication control signal and a time width TC of one part (one chip) of the PN code. As described hereinafter, this spread gain is found from the S/N ratio of the information signal on which the anti-duplication control signal is superimposed, in this case the SIN ratio of the video signal.
For example, when the S/N ratio of the video signal on which the anti-duplication control signal is superimposed is 50 dB, the anti-duplication control signal which is spectrally spread and superimposed on the video signal must be superimposed at a lower level than 50 dB, which is the S/N ratio of the video signal. Also, in order to detect the anti-duplication control signal superimposed on the video signal, its S/N ratio must be sufficient for the spectrally spread signal to be fully demodulated. If this S/N ratio is 10 dB, a spread gain of 60 dB (S/N ratio of 50 dB for video signal)+(S/N ratio of 10 dB necessary for detection) is required. In this case, the PN code length per bit of the anti-duplication control signal is 1 million.
The method used in the recording device to detect the PN code superimposed on the video signal uses a multifilter or a sliding correlation. In the former case, detection of the PN code is rapid, but only a short code length can be detected. At present, this code length is of the order of 256, and when the PN code length is 1 million per bit of the anti-duplication control signal, it cannot be detected. In the latter case, PN codes of long length can be detected but the detection takes time. It can thus be appreciated that a considerable time is required to detect a PN code having a length of 1 million.
Moreover, if the superposition level of the spectrally spread anti-duplication control signal is too high, the anti-duplication control signal causes serious visual interference with the video signal.
This invention, in view of the above, aims to permit additional information to be superimposed on a video signal so as to control duplication of the signal without causing its deterioration, and to control such duplication by precisely and rapidly extracting this additional information.
A video duplication control system according to this invention comprises a playback device for playing back a video signal, and a recording device for receiving the video signal output by the playback device and recording it on a recording medium, wherein:
the playback device comprises:
sync signal separating means on the playback side for separating a sync signal from the video signal,
spread code generating means on the playback side for generating a spread code with a start timing based on the sync signal separated by sync signal separating means on the playback side,
spread code inverting means on the playback side for reversing the phase of the spread code from spread code generating means on the playback side with a timing based on the sync signal separated by the separating means on the playback side,
spectrum spreading means for spectrally spreading duplication control information superimposed on the video signal according to the spread code processed by spread code inverting means on the playback side, and
superimposing means for superimposing the duplication control information which has been spectrally spread by the spectrum spreading means on the video signal,
the recording means comprises:
sync signal separating means on the recording side for separating the sync signal from the video signal supplied by the playback means,
spread code generating means on the recording side for generating a spread code with a start timing based on the sync signal separated by sync signal separating means on the recording side,
spread code inverting means on the recording side for reversing the phase of the spread code from spread code generating means on the recording side with a timing based on the sync signal separated by the separating means on the recording side,
reverse spectrum spreading means for performing reverse spectral spread according to the spread code processed by spread code inverting means on the recording side, and extracting the duplication control information superimposed on the video signal supplied by the playback device, and
duplication control means for controlling recording of the video signal on the recording medium based on the duplication control information extracted by the spectrum spreading means.
A video recording medium according to this invention is characterized in that a video signal on which additional information, which has been spectrally spread by a reverse spread code obtained by reversing, with a timing based on a sync signal in the video signal, the phase of a spread code which starts to be generated with a timing based on the sync signal in the video signal and superimposed on the video signal, is recorded on it.
In the duplication control system according to this invention, a spread code starts to be generated with a timing based on the sync signal separated from the video signal in the playback device, and the spread code is phase inverted with a timing based on this sync signal. In this context, phase inversion of the spread code refers to bit inversion where 0""s are replaced by 1""s and 1""s are replaced by 0""s. The additional information is spectrally spread using this inverted spread code and superimposed on the video signal.
In the recording device, a spread code starts to be generated with a timing which is identical to the generation timing in the playback device relative to the video sync signal and based on the video sync signal separated from the video signal supplied by the playback device. Also, the polarity of the spread code generated in the recording device is reversed with a timing based on the video sync signal. In the recording device, this inverse spread code is used to perform reverse spectral spread.
During reverse spectral spread, the generation timing of the spread code used to perform reverse spectral spread must be controlled for the video signal from the playback device so that it is identical to that of the spread code used to perform spectral spread in the playback device.
This generation timing is determined based on the video sync signal separated from the video signal. The generation timing of the spread code generated in the recording device can therefore be adjusted to be the same as the generation timing of the spread code used in the playback device for the video sync signal.
The spread code used to perform spectral spread in the playback device and the spread code used to perform reverse spectral spread in the recording device are phase inverted with a timing based on the video sync signal. For example, by inverting the polarity of the spread code every other field (one vertical interval), a spread code of different polarity in every field is generated.
In the recording device, similarly to the case of the spread code for performing spread, reverse spectral spread is performed using the spread code for performing reverse spectral spread which has been phase inverted such that, for example, its polarity is different in every field, and the additional information which has been spectrally spread and superimposed on the video signal is detected.
During reverse spectral spread, the spread code which has been arranged to have different polarity every field is applied to the video signal comprising the spectrally spread anti-duplication control signal, and integrated. The anti-duplication control signal superimposed on the video signal is thereby extracted. In this case, by applying the spread code which has been arranged to have different polarity in every field to the playback signal, the polarity of the video signal components in the playback signal is reversed every field.
The video signal is a signal having a high correlation between adjacent fields, between frames and between adjacent horizontal scanning lines. Therefore by performing integration as part of the process of reverse spectral spread, video signal components of different polarity in adjacent fields cancel each other out, and are thereby eliminated.
In this way, the additional information which has been spectrally spread and superimposed on the video signal may be detected without affecting high level video signal components, and the detection efficiency of additional information is improved. Moreover due to this improved detection efficiency of the additional information superimposed on the video signal, the spread gain may also be reduced.
When the polarity of the spread code is reversed with a timing based on the video sync signal in the video signal as described above, the polarity (phase) of the additional information superimposed on the video signal is reversed according to the applied spread code. In this case, in a similar way to polarity reversal every field of the color sub-carrier wave in the color video signal and polarity reversal between every horizontal scanning period, in the additional information superimposed on the video signal, brightness variations between adjacent additional information of different polarity are reversed, so the variations are averaged and therefore do not stand out. Consequently, visual interference of the reproduced video due to the superimposed additional information is suppressed.
In the video duplication system according to this invention, the superposition level of spectrally spread additional information supplied to the superimposing means of the playback device is adjusted by level adjusting means.
Therefore when the effect of the additional information superimposed on the video signal can be reduced, for example by inverting the polarity of the spread code or the like, the superposition level of the additional information may be increased. By increasing the superposition level, the efficiency of detecting the additional information in the recording device may be further improved.
Regarding the video recording medium according to this invention, in the recording device which receives the video signal played back from this video recording medium, by performing reverse spectral spread using a spread code which starts with the same timing and of which the timing is inverted with the same timing relative to the video signal, as that of the spread code used for spectral spread of the additional information superimposed on the video signal recorded on the medium, the efficiency of detecting the additional information is improved as described hereabove.
Further, the polarity of the spread code is reversed with a timing based on the video sync signal, and this inverted spread code is used for performing spectral spread. Hence as described hereabove, in the additional information superimposed on the video signal, brightness variations between adjacent additional information of different polarity are averaged and do not stand out. Consequently, visual interference of the reproduced video due to the superposition of additional information on the video signal is suppressed.
Therefore, even in the case of a recording medium on which a video signal is recorded wherein spectrally spread additional information is superimposed on the video signal to prevent improper duplication, a good quality video can be obtained.